The invention relates to a method of detecting a magnetic preorientation in mechanical parts such as motor rotors; to the application of this method when magnetizing these parts; and to an associated device for magnetizing the parts according to these methods.
Rotors for stepping motors consist of a support with a shait and the actual tube magnet. The tube magnet--also called the "rotor"--is usually magnetized to have two to twelve poles; i.e., there are, in circumferential direction, alternately up to six north and six south poles which extend the entire axial length of this part.
To make handling easier, such a rotor with a tube magnet should be magnetized only immediately before installation in the stator. They are magnetized in a device consisting, for example, of a single copper rod winding having six turns within a laminated iron core. The so-called rotor or tube is inserted into the middle of the lamination pack for magnetization and the winding is energized by a capacitor discharge.
In order to obtain optimal magnetization, the manufacturer of the magnet material forming the tube usually magnetically preorients the pole areas near the surface. Because of this preorientation, the rotor must be rotated to a correct pole pitch position inside the magnetizing device prior to the actual magnetization.
In practice, the rotor tube magnet consists, in particular, of barium ferrite which can be powder-metallurgically molded into appropriate parts by pressing and sintering. To increase the remanence of the magnet material, a magnetic field is applied already during the powder-metallurgical manufacturing process in such a manner that the specified magnetic preferred directions are formed in the part.
Since the magnetically preoriented rotor zones are not physically visible, special procedures are needed to rotate the rotors into the correct pitch position prior to full magnetization.
Conventionally, this is done by adding an additional impedance in series with the magnetizing winding so that, at first, a slowly rising excitation current causes the free-turning rotor to rotate into the correct position. The main pulse for the actual magnetization is then applied to the winding.
There is a problem with this magnetizing technique in that dirt particles may adversely affect the freedom of rotation of the rotor in the magnetizing device so that, in many cases, magnetization will not be optimal.